Wow, BBC News reports that the atomic clock, the time-keeping device that governs all aspects of our lives, is celebrating its 50th year. The first accurate caesium atomic clock was developed at the NPL in 1955 by Dr Louis Essen.

Atomic clocks form the standard for Coordinated Universal Time (UTC), which governs legal time-keeping globally. The clocks are vital for rafts of technologies, such as global satellite navigation (GPS), coordinating packets of data which are transferred across the internet, and mobile telephony.Even London's Big Ben relies on atomic clocks to keep it right.

Using a beam of caesium-133 atoms, generated from a special type of oven, the best present-day atomic clocks are able to keep time to within a 10th of a billionth of a second a day. They do this by counting time based on the way cooled-down caesium atoms jump back and forth between different energy levels. This occurs at microwave frequencies, with nearly 9.2 billion jumps making up the interval of time known as the second.

And here is an page on the history of atomic clocks at NIST (US National Institute of Standards and Technology).